Vapor compression type refrigerating machine

ABSTRACT

A bypass valve  81  is opened until a predetermined period of time elapses after compressors  10   a   , 10   b  are stopped so as to equalize the pressure of a refrigerant circuit on a condenser  20  side with the pressure of a refrigerant circuit on an evaporator  30  side and, after the bypass valve  81  is closed, at least either of a refrigerant circuit  91  connecting to the compressor  10   a and a refrigerant circuit  92  connecting to the compressor  10   b  is opened by opening a three-way valve  90  so that the refrigerant circuit on the condenser  20  side is made to communicate with the refrigerant circuit on the evaporator  30  side via the compressor  10  whereby, as the pressure equalized state can be maintained, it is possible to prevent the accumulation of a large amount of refrigerating machine oil on suction sides of the compressors  10  while the compressors  10  are stopped, thereby making it possible to prevent damage to the compressors  10  due to excessive compression when activated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to, among refrigerating machines in which heat on a low temperature side is moved to a high temperature side, a vapor compression type refrigerating machine having a plurality of compressors which is effective when applied, in particular, to an ejector cycle.

2. Description of the Related Art

The ejector cycle is a cycle used in a vapor compression type refrigerating machine in which the pressure of a refrigerant is reduced by an ejector so that the refrigerant is allowed to expand, vapor-phase refrigerant that has been vaporized by an evaporator is sucked into the ejector, and the suction pressure of the compressor is increased by converting expansion energy into pressure energy (for example, refer to the Japanese Unexamined Patent Publication No. 6-11197).

Incidentally, in a vapor compression type refrigerating machine in which the pressure of a refrigerant is reduced, in an isenthalpic fashion, by a pressure reducing unit such as an expansion valve (hereinafter, referred to as an expansion valve cycle), the refrigerant flowing out from the expansion valve flows into the evaporator, whereas in the ejector cycle, the refrigerant flowing out from the ejector flows into a vapor-liquid separator, and a liquid phase refrigerant resulting from separation by the vapor-liquid separator is supplied into the evaporator while a vapor phase refrigerant resulting from separation by the vapor-liquid separator is sucked into the compressor.

Namely, the expansion valve cycle provides a flow of refrigerant in which the refrigerant circulates from the compressor back to the compressor via the condenser, the expansion valve, and the evaporator sequentially in that order, whereas the ejector cycle provides two flows of refrigerant; one is a flow of refrigerant in which the refrigerant circulates from the compressor back to the compressor via the condenser (a high pressure side heat exchanger), the ejector, and the vapor-liquid separator sequentially in that order, and the other is a flow of refrigerant in which the refrigerant circulates from the vapor-liquid separator back to the vapor-liquid separator via the evaporator and the ejector sequentially in that order.

Then, in the ejector cycle, as the refrigerant in a saturated state flows into a low pressure side heat exchanger, if a low pressure side heat exchanger whose size is the same as that of the low pressure side heat exchanger used in the expansion valve cycle is used in the ejector cycle, the amount of liquid phase refrigerant flowing through the low pressure side heat exchanger becomes larger than that in the expansion valve cycle, and therefore, the amount of refrigerant to be sealed in the cycle must be increased, compared with the expansion valve cycle.

While the amount of a refrigerating machine oil that is mixed in the refrigerant needs to be increased in association with the increase in the amount of refrigerant, in the event that the amount of refrigerating machine oil that is mixed in the refrigerant is increased, the amount of refrigerating machine oil mixed in the refrigerant discharged from the compressor is inevitably increased.

Incidentally, the refrigerating machine oil is a lubricating oil which lubricates sliding parts and bearings within the compressor.

In addition, in the event that the refrigerant that contains a large amount of refrigerating machine oil flows into the heat exchanger such as the high pressure side heat exchanger and the low pressure side heat exchanger, the refrigerating machine oil whose kinematic viscosity is larger than the refrigerant adheres to an internal wall of the heat exchanger to thereby decrease the heat exchange efficiency of the heat exchanger. Thus, it is a normal practice to provide an oil separator for separating the refrigerating machine oil mixed in the refrigerant on a discharge side of the compressor, that is, a refrigerant inlet side of the high pressure side heat exchanger, so that refrigerating machine oil separated by the oil separator is returned to a suction side of the compressor via an oil return circuit which is constituted as a restriction unit such as a capillary tube.

In addition, in a vapor compression type refrigerating machine having a plurality of compressors, as the vapor compression type refrigerating machine is operated while a high load operation mode, in which all the compressors are in operation, and a low load operation mode, in which any of the plurality of compressors is in operation are changed over, in order to prevent a high pressure refrigerant discharged from the compressor from flowing into the compressors which are not in operation, check valves 10 c, 10 d are provided, as shown in FIG. 2, along refrigerant circuits which connect to discharge sides of the respective compressors 10 a, 10 b.

In the refrigerating machine shown in FIG. 2, that is, the refrigerating machine including the plurality of compressors 10 a, 10 b arranged in parallel relative to the flow of refrigerant for sucking in and compressing a refrigerant, a high pressure side heat exchanger 20 for removing heat from a high pressure refrigerant discharged from the compressors 10 a, 10 b, a low pressure side heat exchanger 30 for vaporizing a low pressure refrigerant and absorbing heat therefrom, an oil separator 70 provided on a refrigerant inlet side of the high pressure side heat exchanger 20 for separating and extracting a refrigerating machine oil mixed in the refrigerant, and an oil return circuit 71 for returning the refrigerating machine oil so separated and extracted by the oil separator 70 to the suction sides of the compressors 10 a, 10 b, a difference in pressure between a pressure remaining on the high pressure side heat exchanger 20 side and a pressure remaining on the low pressure side heat exchanger 30 side is large immediately after all the plurality of compressors 10 a, 10 b are stopped, and as the check valves 10 c, 10 d are provided on the discharge sides of the compressors 10 a, 10 b, the refrigerating machine oil separated and extracted by the oil separator 70 returns to the suction sides of the compressors 10 a, 10 b via the oil return circuit 71.

Due to this, as the refrigerating machine oil that has been separated and extracted by the oil separator 70 continues to return to the suction sides of the compressors 10 a, 10 b via the oil return circuit 71 until the pressures on the high and low pressure sides become equal, a large amount of refrigerating machine oil is accumulated on the suction sides of the compressors 10 a, 10 b.

Then, when the compressors 10 a, 10 b are activated with the large amount of refrigerating machine oil being accumulated on the suction sides of the compressors 10 a, 10 b, as the compressors 10 a, 10 b pick up a large amount of refrigerating machine oil, which is liquid, an excessively compressed state results from liquid compression, and it is highly probable that the compressors 10 a, 10 b are damaged.

In contrast to this, as shown in FIG. 3, there are provided a bypass circuit 80 for establishing a communication between the refrigerant circuit on the high pressure side heat exchanger 20 side and the refrigerant circuit on the low pressure side heat exchanger 30 side and a bypass valve 81 for opening and closing the bypass circuit 80, whereby, when the plurality of compressors 10 a, 10 b are stopped, the bypass valve 81 is opened. This construction provides, however, another problem as described below.

Namely, in addition to the difference in pressure, there also exists a large difference in temperature between the refrigerant circuit on the high pressure side heat exchanger 20 side and the refrigeration cycle on the low pressure side heat exchanger 30 side.

As this occurs, while the difference in pressure between the refrigerant circuit on the high pressure side heat exchanger 20 side and the refrigeration cycle on the low pressure side heat exchanger 30 side can be eliminated to provide an equalized pressure therebetween within a relatively short period of time (for example, in the order of 30 seconds) by opening the bypass valve 81, as the high pressure side heat exchanger 20 and the low pressure side heat exchanger 30 have a relatively large heat capacity, even in the event that the pressures of the refrigerant circuit on the high pressure side heat exchanger 20 side and the refrigeration cycle on the low pressure side heat exchanger 30 side become equal, the difference in temperature between the refrigerant circuit on the high pressure side heat exchanger 20 side and the refrigeration cycle on the low pressure side heat exchanger 30 side cannot be reduced in the same way as the pressure difference is reduced.

Consequently, when the bypass valve is closed after the pressure of the refrigerant circuit on the high pressure side heat exchanger 20 side and the pressure of the refrigerant circuit on the low pressure side heat exchanger 30 side become equal by opening the bypass valve 81, there is caused, as shown in FIG. 4, a difference in pressure between the refrigerant circuit on the high pressure side heat exchanger 20 side and the refrigerant circuit on the low temperature side heat exchanger 30 side due to the difference in temperature between the refrigerant circuit on the high pressure side heat exchanger 20 side and the refrigerant circuit on the low temperature side heat exchanger 30 side.

Due to this, in order to make sufficiently uniform the pressure of the refrigerant circuit on the high pressure side heat exchanger 20 side and the pressure of the refrigerant circuit on the low temperature side heat exchanger 30 side, it is desirable to keep the bypass valve 81 open until the compressors 10 a, 10 b, that is, the vapor compression type refrigerating machine is re-activated after the refrigerating machine has been stopped.

On the other hand, in order to prevent the occurrence of a problem with operation of the vapor compression type refrigerating machine even in the event that the bypass valve 81 fails, a normally-closed type valve is desirably adopted for the bypass valve 81.

Note that, in electromagnetic valves or the like, for example, the normally-closed type valve means a valve which closes when not energized and opens when energized.

When adopting a normally-opened valve as the bypass valve 81, however, as the bypass valve 81 needs to be energized until the vapor compression type refrigerating machine is re-activated after it has been stopped, the dark current, that is, the current consumed while the vehicle is stopped increases.

SUMMARY OF THE INVENTION

The invention was made in view of the situations and a first object thereof is to provide a novel vapor compression type refrigerating machine which is different from conventional ones, and a second object of the invention is to prevent damage to a compressor due to excessive compression when the refrigerating machine is activated.

With a view to attaining the objects, according to one aspect of the invention, there is provided a vapor compression type refrigerating machine for moving heat on a low temperature side to a high temperature side comprising a plurality of compressors (10 a, 10 b) arranged in parallel relative to the flow of a refrigerant for sucking in and compressing a refrigerant, a high-pressure side heat exchanger (20) for removing heat from a highly pressurized refrigerant discharged from the compressors (10 a, 10 b), a low-pressure side heat exchanger (30) for absorbing heat by vaporizing a low pressure refrigerant, an oil separator (70) provided on a refrigerant inlet side of the high-pressure heat exchanger (20) for separating and extracting a refrigerating machine oil mixed in the refrigerant, an oil return circuit (71) for returning the refrigerant so separated and extracted by the oil separator (70) to suction sides of the compressors (10 a, 10 b), a bypass circuit (80) for establishing a communication between a refrigerant circuit on a high-pressure side heat exchanger (20) side and a refrigerant circuit on a low-pressure side heat exchanger (30) side, a bypass valve (81) for opening and closing the bypass circuit (80), a compressor valve (90) for opening and closing refrigerant circuits (91, 92) which connect to the compressors (10 a, 10 b), respectively, and a control unit (100) for controlling both the valves (81, 90) such that the bypass valve (81) is kept open until a predetermined period of time has elapsed after the plurality of compressors (10 a, 10 b) were stopped and that, after the predetermined period of time has elapsed, the bypass valve (81) is closed while the compressor valve (90) is opened.

Then, according to the invention, the bypass valve (81) is kept open until the predetermined period of time has elapsed after the compressors (10 a, 10 b) were stopped, so that the pressure of the refrigerant circuit on the high-pressure side heat exchanger (20) side and the pressure of the refrigerant circuit on the low-pressure side heat exchanger (30) side are made equal, and after the bypass valve (81) is closed, the compressor valve (90) is opened, so that the refrigerant circuit on the high-pressure side heat exchanger (20) side is made to communicate with the refrigerant circuit on the low-pressure side heat exchanger (30) side via the compressors (10 a, 10 b). Thus, even in the event that there is a big difference in temperature between the high-pressure side heat exchanger (20) side and the low-pressure side heat exchanger (30) side, it is possible to prevent the generation of a difference in pressure to cause the refrigerating machine oil to flow between the refrigerant circuit on the high-pressure side heat exchanger (20) side and the refrigerant circuit on the low-pressure side heat exchanger (30) side due to the difference in temperature.

Consequently, as the accumulation of a large amount of refrigerating machine oil on the suction sides of the compressors (10 a, 10 b), while the compressors (10 a, 10 b) are stopped, can be prevented, it is possible to prevent a risk that the compressors (10 a, 10 b) are damaged due to excessive compression when the refrigerating machine is activated.

According to another aspect of the invention, there is provided a vapor compression type refrigerating machine comprising a plurality of compressors (10) arranged in parallel relative to the flow of a refrigerant for sucking in and compressing a refrigerant, a high-pressure side heat exchanger (20) for removing heat from a highly pressurized refrigerant discharged from the compressors (10 a, 10 b), a low-pressure side heat exchanger (30) for absorbing heat by vaporizing a low pressure refrigerant, an ejector (40) having a nozzle (41) for converting a pressure energy of the highly pressurized refrigerant that flows out from the high-pressure side heat exchanger (20) into a velocity energy so as to reduce the pressure of the refrigerant for expansion and pressure increasing portions (42, 43) for sucking in a vapor-phase refrigerant vaporized by a high-speed flow of refrigerant injected from the nozzle (41) at the low-pressure side heat exchanger (30) and mixing the refrigerant injected from the nozzle (41) with the refrigerant sucked in from the low-pressure side heat exchanger (30) so as to convert the velocity energy into a pressure energy to thereby increase the pressure of the refrigerant, a vapor-liquid separator (50) for separating the refrigerant that has flowed out from the ejector (40) into a vapor-phase refrigerant and a liquid-phase refrigerant in which an outlet for the vapor-phase refrigerant is connected to suction sides of the compressors (10 a, 10 b) and an outlet for the liquid-phase refrigerant is connected to the low-pressure side heat exchanger (30), an oil separator (70) provided on a refrigerant inlet side of the high-pressure heat exchanger (20) for separating and extracting a refrigerating machine oil mixed in the refrigerant, an oil return circuit (71) for returning the refrigerant so separated and extracted by the oil separator (70) to the suction sides of the compressors (10 a, 10 b), a bypass circuit (80) for establishing a communication between a refrigerant circuit on a high-pressure side heat exchanger (20) side and a refrigerant circuit on a low-pressure side heat exchanger (30) side, a bypass valve (81) for opening and closing the bypass circuit (80), a compressor valve (90) for opening and closing refrigerant circuits (91, 92) which connect to the compressors (10 a, 10 b), respectively, and a control unit (100) for controlling both the valves (81, 90) such that the bypass valve (81) is kept open until a predetermined period of time has elapsed after the plurality of compressors (10 a, 10 b) were stopped and that after the predetermined period of time has elapsed, the bypass valve (81) is closed while the compressor valve (90) is opened.

Then, according to the invention, the bypass valve (81) is kept open until the predetermined period of time has elapsed after the compressors (10 a, 10 b) were stopped, so that the pressure of the refrigerant circuit on the high-pressure side heat exchanger (20) side and the pressure of the refrigerant circuit on the low-pressure side heat exchanger (30) side are made equal, and after the bypass valve (81) is closed, the compressor valve (90) is opened, so that the refrigerant circuit on the high-pressure side heat exchanger (20) side is made to communicate with the refrigerant circuit on the low-pressure side heat exchanger (30) side via the compressors (10 a, 10 b). Thus, even in the event that there is a big difference in temperature between the high-pressure side heat exchanger (20) side and the low-pressure side heat exchanger (30) side, it is possible to prevent the generation of a difference in pressure to cause the refrigerating machine oil to flow between the refrigerant circuit on the high-pressure side heat exchanger (20) side and the refrigerant circuit on the low-pressure side heat exchanger (30) side due to the difference in temperature.

Consequently, as the accumulation of a large amount of refrigerating machine oil on the suction sides of the compressors (10 a, 10 b) while the compressors (10 a, 10 b) are stopped can be prevented, it is possible to prevent a risk that the compressors (10 a, 10 b) are damaged due to excessive compression when the refrigerating machine is activated.

According to the invention, the compressor valve (90) opens and closes the refrigerant circuits (91, 92) which connect to discharge sides of the compressors (10 a, 10 b).

Incidentally, parenthesized reference numerals imparted to the respective units above correspond to specific examples of units that are described in an embodiment of the invention that will be described later on.

The present invention will be more fully understood with reference to the accompanying drawings and a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an exemplary diagram illustrating an ejector cycle according to an embodiment of the invention; FIG. 2 is an exemplary diagram illustrating an ejector cycle according to a related art;

FIG. 3 is an exemplary diagram illustrating an ejector cycle according another related art; and

FIG. 4 is a graph illustrating pressure behaviors of the ejector cycles according to the related arts.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In an embodiment of the invention, an ejector cycle according to the invention is applied to a vapor compression type refrigerating machine which needs to decrease the temperature in a showcase for preserving foods and drinks in cooled and frozen conditions or a refrigerator of a refrigerated vehicle for transporting foods and drinks that are preserved in cooled and frozen conditions lower than the temperature of an air conditioner.

Compressors 10 a, 10 b suck in and compress a refrigerant by obtaining power from an electric motor, and these two compressors 10 a, 10 b are arranged in parallel relative to the flow of a refrigerant. Note that when the compressors 10 a, 10 b are referred to collectively, they are described as the compressor 10, whereas when the respective compressors need to be described individually, they are described as the compressor 10 a or the compressor 10 b.

A condenser 20 is a high-pressure side heat exchanger constituting a radiator for implementing a heat exchange between a high-temperature, high-pressure refrigerant discharged from the compressor 10 and outside air so as to cool and condense the refrigerant, and an evaporator 30 is a low-pressure side heat exchanger for implementing a heat exchange between air sent into a refrigerator and a low-pressure refrigerant so as to vaporize a liquid-phase refrigerant to thereby exhibit a refrigerating capacity.

An ejector 40 is an ejector for sucking in a vapor-phase refrigerant which is vaporized at the evaporator 30 by reducing the pressure of the refrigerant that has flowed out from the condenser 20 for expansion and converting an expansion energy into a pressure energy so as to increase the suction pressure of the compressor 10.

Then, the ejector 40 includes a nozzle 41 for converting the pressure energy of the high-pressure refrigerant that flows thereinto into a velocity energy so as to reduce the pressure of the refrigerant, in an isenthalpic fashion, a fixing portion 42 for sucking in the vapor-phase refrigerant that is vaporized at the evaporator 30 through an entrainment action by a high-speed flow of refrigerant injected from the nozzle 41 for mixing with the flow of refrigerant injected from the nozzle 41 and a diffuser 43 for mixing the refrigerant injected from the nozzle 41 with the refrigerant sucked in from the evaporator 30 so as to convert the velocity energy into a pressure energy to thereby increase the pressure of the refrigerant.

As this occurs, at the mixing portion 42, as a drive flow and a suction flow mix with each other so that a sum of the kinetic momentum of the drive flow and the kinetic momentum of the suction flow is preserved, the pressure (the static pressure) of the refrigerant is also increased at the mixing portion 42.

On the other hand, at the diffuser 43, as the velocity energy (the dynamic pressure) of the refrigerant is converted into a pressure energy (a static pressure) by gradually expanding the cross-sectional area of a passageway, the pressure of the refrigerant is increased at both the mixing portion 42 and the diffuser 43 in the ejector 40. Hence, hereinafter, the mixing portion 42 and the diffuser 43 are generally referred to as a pressure increasing portion.

Incidentally, in this embodiment, in order to accelerate the velocity of the refrigerant injected from the nozzle 41 to a velocity equal to or faster than the sonic velocity, while an Laval nozzle (refer to Fluid Engineering (Tokyo University Publication Association)) having a throat portion where the area of the passage is reduced to the minimum at a position along the length of the passageway is adopted, of course, it goes without saying that a tapered nozzle may be adopted.

In addition, the vapor-liquid separator 50 is a vapor-liquid separating unit into which the refrigerant that has flowed out from the ejector 40 flows and which is adapted to store the refrigerant that has so flowed in by separating the refrigerant into a vapor-phase refrigerant and a liquid-phase refrigerant, and an outlet for the vapor-phase refrigerant of the vapor-liquid separator 50 is connected to a suction side of the compressor 10, whereas an outlet for the liquid-phase refrigerant thereof is connected to the evaporator 30 side.

A variable restriction unit 60 is an expansion valve which is provided at a position along the refrigerant passageway between the condenser 20 and the ejector 40, that is, upstream of the nozzle 41 with respect to the flow of refrigerant for reducing the pressure of the highly-pressurized refrigerant that has flowed out from the condenser 20 to a vapor-liquid two-phase area for expansion. This variable restriction unit 60 is such as to control the opening of restriction so that the degree of superheating of refrigerant on the refrigerant outlet side of the evaporator 30 resides within a predetermined range (for example, 0.1 deg to 10 deg) and has a similar construction to that of a known external pressure equalizing type expansion valve.

To be specific, the variable restriction unit 60 is such as to include a valve element 61 for varying the opening of the restriction, a film-like diaphragm 63 constituting a back pressure compartment 62 where an internal pressure varies by sensing the refrigerant temperature on the refrigerant outlet side of the evaporator 30, a connecting rod 64 which connects the valve element 61 to the diaphragm 63 so as to transfer the displacement of the diaphragm 63, a spring 65 adapted to apply a spring pressure in a direction in which the volume of the back pressure compartment 62 is reduced and an external equalizer pipe 67 for introducing the pressure of the refrigerant on the refrigerant outlet side of the evaporator 30 into a pressure compartment 66 which is situated opposite to the back pressure compartment 62 across the diaphragm 63.

Note that the back pressure compartment 62 communicates with a temperature sensing tube 62 a for sensing the temperature of refrigerant on the refrigerant outlet side of the evaporator 30, whereby the temperature of refrigerant on the refrigerant outlet side of the evaporator 30 is transmitted to the back pressure compartment 62 via the temperature sensing tube 62 a.

Due to this, the variable restriction unit 60 reduces the opening of restriction thereof so as to increase the velocity of the drive flow injected from the nozzle 41 to thereby increase the suction flow or the amount of refrigerant circulating through the evaporator 30 when the pressure in the evaporator 30, that is, the heat load in the evaporator 30 increases, whereby the degree of superheating of refrigerant on the outlet side of the evaporator 30 increases. On the contrary, when the pressure within the evaporator 30 decreases, whereby the degree of superheating of refrigerant on the outlet side of the evaporator 30 decreases, the variable restriction unit 60 increases the opening of restriction thereof so as to decrease the velocity of the drive flow injected from the nozzle 41 to thereby decrease the amount of refrigerant which circulates through the evaporator 30.

An oil separator 70 is such as to separate and extract a refrigerating machine oil mixed in the refrigerant, and this oil separator 70 is provided on a refrigerant inlet side of the condenser 20.

Note that, as oil separators, there are a centrifugal separation method for separating a refrigerating machine oil from a refrigerant by rotating, at high speed, the refrigerant in which the refrigerating machine oil is mixed and a collision separation method for separating a refrigerating machine oil from a refrigerant by causing the refrigerant in which the refrigerating machine oil is mixed to collide against a wall surface at high speed. In this embodiment, the centrifugal separation system is adopted.

An oil return circuit 71 is a circuit for returning the refrigerating machine oil separated and extracted by the oil separator 70 to the suction side of the compressor 10. This oil return circuit 71 is made up of a fixed restriction such as a capillary tube (a fine tube) or an orifice whose restriction opening is fixed, and in this embodiment, a capillary tube is adopted.

Note that the oil return circuit 71 is set such that a pressure loss is generated which is substantially equal to a sum of the pressure reduction amount of the nozzle 41 and the pressure reduction amount of the variable restriction unit 60.

A bypass circuit 80 is a refrigerant circuit for establishing a communication between a refrigerant circuit on the condenser 20 side and a refrigerant circuit on the evaporator 30 side, and a bypass valve 81 is a normally-closed electromagnetic valve for opening and closing the bypass circuit 80.

Note that, in this embodiment, a high-pressure side of the bypass circuit 80 is connected to the refrigerant circuit on the condenser 20 side at a position between the condenser 20 and the oil separator 70, whereas a low-pressure side of the bypass circuit 80 is connected to the refrigerant circuit on the evaporator 30 side at a position between the vapor-liquid separator 50 and the evaporator 30.

A three-way valve 90 is a compressor valve for opening and closing refrigerant circuits 91, 92 which connect to the compressors 10 a, 10 b, respectively. The three-way valve 90 is an electric valve for switching the case where the refrigerant circuit 91 connecting to the compressor 10 a is opened whereas the refrigerant circuit 92 connecting to the compressor 10 b is closed, the case where the refrigerant circuit 91 connecting to the compressor 10 a is closed whereas the refrigerant circuit 92 connecting to the compressor 10 b is opened, and the case where the refrigerant circuits 91, 92 are both opened.

Note that, in this embodiment, while the three-way valve 90 is disposed on a merging side of the refrigerant circuits 91, 92, that is, on discharge sides of the compressors 10 a, 10 b, the three-way valve 90 may be disposed on a branching side of the refrigerant circuits 91, 92, that is, the suction sides of the compressors 10 a, 10 b.

Then, the operations of the bypass valve 81 and the three-way valve 90 are controlled by an electronic control unit 100, and signals from rotational speed sensors 101, 102 for detecting the rotational speed of the compressors 10 a, 10 b are inputted into the electronic control unit 100.

Note that the electronic control unit 100 detects whether or not the compressors 10 a, 10 b are stopped based on the rotational speeds, of the compressors 10 a, 10 b, that are detected by the rotational speed sensors 101, 102.

Next, the operation of an ejector cycle will be described briefly.

1. Basic Operation

This operation is an operation mode for generating a refrigerating capacity at the compressor 30.

To be specific, the refrigerant discharged from the compressor 10 is circulated to the condenser 20 side, whereby the pressure of the highly pressurized refrigerant that is cooled at the condenser 20 is reduced in an isenthalpic fashion down to the vapor-liquid two-phase area by the variable restriction unit 60. Thereafter, the pressure of the refrigerant so reduced in pressure is reduced in an isenthalpic fashion by the nozzle 41 of the ejector 40 so that the refrigerant expands, whereby the refrigerant flows into the mixing portion 42 at faster speed than sonic velocity.

As this occurs, in this embodiment, the refrigerant is once boiled at the variable restriction unit 60, and the refrigerant is expanded at an inlet portion of the nozzle 41 so as to restore the pressure, whereby the refrigerant can be boiled at a second-stage nozzle while continuing to generate boiling nucleus. Thus, the boiling of refrigerant at the nozzle 41 can be promoted, thereby making it possible to improve the ejector efficiency ηe by making the drops of refrigerant become minute particles.

Incidentally, the ejector efficiency ηe is defined by using, as a denominator, a product of the mass flow rate Gn of refrigerant which flows through the condenser 20 and a difference in enthalpy Δie between the outlet and inlet of the nozzle 41 and putting, as a numerator, a sum of a refrigerant flow rate Gn indicating to what extent the energy is recovered, as work done, by the compressor 10 and the mass flow rate Ge of refrigerant which flows through the evaporator 30 and a pressure recovery ΔP at the ejector 40.

Note that, in this embodiment, chlorofluorocarbon is used as refrigerant, and the high-pressure side refrigerant pressure, that is, the pressure of refrigerant that flows into the nozzle is made to be equal to or less than the critical pressure of the refrigerant.

On the other hand, as refrigerant vaporized within the evaporator 30 is sucked into the mixing portion 42 by virtue of a pumping action (refer to Japanese Industry Standard (JIS) Z8126, No. 2. 1. 2. 3 and the like) generated in association with the entrainment action of the high-speed refrigerant that has flowed into the mixing portion 42, the refrigerant on the low-pressure side circulates from the vapor-liquid separator 50 back to the vapor-liquid separator 50 via the evaporator 30 and the ejector 40 (the pressure increasing portion) sequentially and in that order.

Then, while the refrigerant (suction flow) sucked in from the evaporator 30 and the refrigerant (drive flow) spouted from the nozzle 41 are being mixed together at the mixing portion 42, the dynamic pressure of the mixed refrigerants is converted into a static pressure by the diffuser 43 and the refrigerant is then returned to the vapor-liquid separator 50.

Note that when the refrigeration load is large as in a case where a large refrigerating capacity is exhibited at the evaporator 30 or a case where the outside temperature is high, the two compressors 10 a, 10 b are both operated, whereas when the refrigeration load is small, only one (for example, the compressor 10 a) of the two compressors 10 a, 10 b is operated.

2. Refrigerating Machine Stop Mode

This operation mode is such as to be executed in a case where the two compressors 10 a, 10 b are both stopped.

To be specific, the electronic control unit 100 continues to energize the bypass valve 81 until a predetermined period of time (for example, 30 sec) has elapsed since the compressors 10 a, 10 b were stopped so as to open the bypass circuit 80, and when the predetermined period of time has elapsed, the electronic control unit 100 cuts off the energization of the bypass valve 81 so as to close the bypass circuit 80 and opens the three-way valve 90, whereby at least one (for example, the refrigerant circuit 91) of the refrigerant circuit 91 connecting to the compressor 10 a and the refrigerant circuit 10 b connecting to the refrigerant circuit 92 is opened.

Next, the function and advantage of the embodiment will be described below.

In this embodiment, the bypass valve 81 is opened until the predetermined period of time has elapsed since the compressors 10 a, 10 b were stopped so that the pressure of the refrigerant circuit on the condenser 20 side and the pressure of the refrigerant circuit on the evaporator 30 side are equalized and, after the bypass valve 81 is closed, the three-way valve 90 is opened so as to open at least either of the refrigerant circuit 91 connecting to the compressor 10 a and the refrigerant circuit 92 connecting to the compressor 10 b to thereby establish a communication between the refrigerant circuit on the condenser 20 side and the refrigerant circuit on the evaporator 30 side via the compressor 10. Thus, even in the event that the difference in temperature between the condenser 20 side and the evaporator 30 side is large, it is possible to prevent the generation of a difference in pressure to cause the refrigerating machine oil to flow between the refrigerant circuit on the condenser 20 side and the refrigerant circuit on the evaporator 30 side due to the difference in temperature.

Namely, the embodiment is such that when the compressors 10 a, 10 b are stopped, firstly, the bypass valve 81 is opened so as to equalize the pressure of the refrigerant circuit on the condenser 20 side with the pressure of the refrigerant circuit on the evaporator 30 side, and thereafter, the refrigerant circuit on the condenser 20 side and the refrigerant circuit on the evaporator 30 side are made to communicate with each other via the refrigerant circuits 91, 92 connecting to compressor 10, whereby the equalized pressure state is maintained.

Consequently, as it is possible to prevent the accumulation of a large amount of refrigerating machine oil on the suction side of the compressor 10 while the compressor is stopped, it is possible to prevent the occurrence of a risk that the compressor 10 is damaged due to excessive compression when activated.

In the embodiment, while the compressors 10 a, 10 bsuck in and compress refrigerant by obtaining power from the electric motor, the invention is not limited thereto, and the compressors 10 a, 10 b may suck in and compress refrigerant by obtaining power from an engine such as an internal combustion engine.

In addition, while, in the embodiment, the invention is applied to the showcase or the like for preserving foods and drinks in cooled and frozen conditions, the application of the invention is not limited thereto, and the invention may be applied to, for example, a vapor compression type refrigerating machine for an air conditioner.

Additionally, while, in the embodiment, the external pressure equalizing type temperature expansion valve is adopted as the variable restriction unit 60, an internal pressure equalizing type temperature expansion valve may be adopted as the variable restriction unit 60.

In addition, while, in the embodiment, the variable restriction unit 60 and the nozzle 41 are provided separately, the invention is not limited thereto, and for example, the variable restriction unit 60 and the nozzle 41 may be integrated into a single unit.

Additionally, in Description of the Related Art, while the description is made by comparing the expansion valve cycle with the ejector cycle, the aforesaid problems also occur, more or less, in the expansion valve cycle, and therefore, the invention can be applied to the expansion valve cycle.

In addition, while, in the embodiment, the compressor valve is made up of the three-way valve 90, the invention is not limited thereto, and the compressor valve may be made up by disposing an electromagnetic switching valve along the length of, for example, each of the refrigerant circuit 91 connecting to the compressor 10 a and the refrigerant circuit 92 connecting the compressor 10 b.

While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention. 

1. A vapor compression type refrigerating machine, for moving heat on a low temperature side to a high temperature side, comprising: a plurality of compressors arranged in parallel relative to the flow of a refrigerant for sucking in and compressing a refrigerant; a high-pressure side heat exchanger for removing heat from a highly pressurized refrigerant discharged from the compressors; a low-pressure side heat exchanger for absorbing heat by vaporizing a low pressure refrigerant; an oil separator provided on a refrigerant inlet side of the high-pressure heat exchanger for separating and extracting a refrigerating machine oil mixed in the refrigerant; an oil return circuit for returning the refrigerant so separated and extracted by the oil separator to suction sides of the compressors; a bypass circuit for establishing a communication between a refrigerant circuit on a high-pressure side heat exchanger side and a refrigerant circuit on a low-pressure side heat exchanger side; a bypass valve for opening and closing the bypass circuit; a compressor valve for opening and closing refrigerant circuits which connect to the compressors, respectively; and a control unit for controlling both the valves such that the bypass valve is kept open until a predetermined period of time elapses after the plurality of compressors are stopped and that, after the predetermined period of time has elapsed, the bypass valve is closed, while the compressor valve is opened.
 2. A vapor compression type refrigerating machine as set forth in claim 1, wherein the compressor valve opens and closes the refrigerant circuits which connect to discharge sides of the compressors.
 3. A vapor compression type refrigerating machine comprising: a plurality of compressors (10) arranged in parallel relative to the flow of a refrigerant for sucking in and compressing a refrigerant; a high-pressure side heat exchanger for removing heat from a highly pressurized refrigerant discharged from the compressors; a low-pressure side heat exchanger for absorbing heat by vaporizing a low pressure refrigerant; an ejector having a nozzle for converting pressure energy of the highly pressurized refrigerant that flows out from the high-pressure side heat exchanger into velocity energy so as to reduce the pressure of the refrigerant for expansion thereof and a pressure increasing portion for sucking in a vapor-phase refrigerant vaporized by a high-speed flow of refrigerant injected from the nozzle at the low-pressure side heat exchanger and mixing the refrigerant injected from the nozzle with the refrigerant sucked in from the low-pressure side heat exchanger so as to convert the velocity energy into a pressure energy to thereby increase the pressure of the refrigerant; a vapor-liquid separator for separating the refrigerant that has flowed out from the ejector into a vapor-phase refrigerant and a liquid-phase refrigerant in which an outlet for the vapor-phase refrigerant is connected to suction sides of the compressors and an outlet for the liquid-phase refrigerant is connected to the low-pressure side heat exchanger; an oil separator provided on a refrigerant inlet side of the high-pressure heat exchanger for separating and extracting a refrigerating machine oil mixed in the refrigerant; an oil return circuit for returning the refrigerant so separated and extracted by the oil separator to the suction sides of the compressors; a bypass circuit for establishing communication between a refrigerant circuit on a high-pressure side heat exchanger side and a refrigerant circuit on a low-pressure side heat exchanger side; a bypass valve for opening and closing the bypass circuit; a compressor valve for opening and closing refrigerant circuits which connect to the compressors, respectively; and a control unit for controlling both the valves such that the bypass valve is kept open until a predetermined period of time elapses after the plurality of compressors are stopped and that after the predetermined period of time has elapsed, the bypass valve is closed, while the compressor valve is opened.
 4. A vapor compression type refrigerating machine as set forth in claim 3, wherein the compressor valve opens and closes the refrigerant circuits which connect to discharge sides of the compressors. 